1. Field of the Invention
This invention pertains to the art of speed control of an internal combustion engine which is desired to operate at at least two separate speeds, and in which loss of oil pressure control is also desired.
While the invention may be employed in connection with engines in various environments, emphasis herein will be given to its usefulness in connection with a transport refrigeration unit such as are typically used on truck trailers and other containers for goods which require either heating or cooling or both.
2. Description of the Prior Art
The use of internal combustion engines which operate continuously at one speed, and then shift to another speed in accordance with the requirements of refrigeration or heating are well known. One arrangement for obtaining different speeds in connection with a throttle lever on an engine is disclosed in U.S. Pat. No. 4,117,740. In the common contemporary commercial arrangements the speed control lever for the engine is actuated by an electrically powered solenoid in accordance with operating conditions of the refrigeration system. Such solenoids are, in a sense, relatively large in that they require 60-70 watts continuously and weigh more than 6 pounds. An arrangement which eliminates these relatively large, and fairly costly, electric actuators in favor of much smaller control solenoids is desirable. One aim of my invention is to accomplish this.
The engines for typical transport refrigeration systems may or may not have a loss of engine oil pressure control arrangement. One typical loss of oil pressure control arrangement uses an oil pressure sensitive switch and a thermal time delay. The time delay arrangement is to prevent a nuisance shutdown, such as from a momentary or intermittent loss of oil pressure, and may, for example, comprise a solder pot with a heater. With such an arrangement, at normal ambient temperature conditions, the loss of oil pressure will result in a safety shutdown in about 45 seconds after the condition is detected. While this is a relatively long period, it does not necessarily result in damage to the engine since the pressure sensitive switch operates at some minimum pressure such as 15 psi. In some cases this feature causes undesired shutdown or requires engine speed be maintained higher than the preferred minimum speed. A more significant disadvantage of this particular arrangement is that since it requires heating of the solder or other thermally sensitive element to permit the safety shutdown to occur, at low ambient temperature such as below 0.degree. F., the safety shutdown arrangement may not work at all, even with a total loss of oil pressure. It is another aim of my invention to avoid these disadvantages, and more specifically to build a loss of oil pressure control into the speed control and in an arrangement in which continued engine operation can occur with a moderate, acceptable loss of oil pressure, and an engine shutdown will occur upon further degradation of the oil pressure.